The present invention relates to a method for manufacturing a semiconductor device wherein an insulating film is formed in a field region for element isolation.
In a semiconductor device using silicon for semiconductors, especially in a MOS semiconductor integrated circuit, a thick insulating film is formed in the field region between the elements in order to prevent unsatisfactory isolation due to parasitic channels and to reduce the parasitic capacitance to the minimum. Selective oxidation is well known as a method for element isolation. According to this selective oxidation method, the element forming region is covered with an antioxidant mask, typically, a silicon nitride film. Then, oxidation is performed at a high temperature to selectively form a thick oxide film on the field region. However, with this selective oxidation method, during the oxidation at high temperature, the field oxide film undercuts the edge of the silicon nitride film in the form of a bird's beak. This causes a size error in the element forming region and interferes with high integration of integrated circuits. Furthermore, in the conventional selective oxidation method, a step of about half the thickness (about 0.7 to 1.0.mu.) of the field oxide film is formed in the field region and the element forming region after the formation of the field oxide film. This step remains in the following process and results in the degradation in the precision of lithography and the reliability of the metal wiring. The conventional selective oxidation method further involves a long field oxidation time. This results in various problems such as undesirable diffusion of impurities from the channel stopper, or formation of defective crystals such as dislocation in the silicon substrate which is caused by the stresses due to the growth of the oxide film, as well as due to a difference in thermal expansion coefficient between the silicon nitride film and the silicon substrate.
In order to avoid the defects as described above involved in the element isolation method by the selective oxidation, a method is disclosed in Japanese Patent Publication (KOKAI) No. 11,792/75, wherein a mask is formed on a semiconductor substrate, a groove of predetermined depth is formed in the substrate by etching, an insulating film is formed on the entire surface of the structure by the CVD process to a thickness sufficient to fill the groove, and the mask is then removed by etching to simultaneously remove the insulating film on the mask while leaving the insulating film in the groove only to provide it as an element isolation layer. According to this method, the process may be performed at a relatively low temperature, the adverse effects such as thermal distortion of the substrate may be reduced to the minimum, the formation of the bird's beaks is restricted, and the lateral diffusion of the impurity layer of the channel stopper may be prevented. However, a recess may be formed between the side walls of the groove and the side surfaces of the remaining insulating film. A void may be formed within the remaining insulating film. The formation of such a void become more prominent when the depth of the groove is relatively large in comparison with the width of the groove, for example, when the depth of the groove is more than twice the width.